1. Field of the Invention
The present invention relates to a method of forming a conductive thick film pattern on a circuit board and a conductive ink used therein.
2. Description of the Prior Art
Hitherto, as the most popular method for forming a thick film pattern on a circuit board, screen printing is known. By varying the screen mesh, possibility of printing pattern width of 30 .mu.m is reported by Ikegami, Goto and Tokumaru in "Patterning Technology in Hybrid Microelectronics," Electronic Materials, May 1989, p. 44, but since wires are broken due to impedance of linearity of pattern and lack owing to the effects of mesh, the limit pattern width is about 100 .mu.m as mass produced industrial products by screen printing.
By contrast, the Japanese Examined Patent Publication No. 55-36512 discloses a thick film printing method by offset printing. Here, the rubber hardness of soft material is 30 degree or less, and it is difficult to reproduce the pattern precisely in the fine-line pattern of 100 .mu.m or smaller pitch.
On the other hand, some of the screen printing pastes commercially available as conductive pastes contain ethyl cellulose resin or acrylic resin in the binders. In the Japanese Un-examined Patent Publication No. 2-228375, an acrylic resin with molecular weight of 50,000 to 500,000 is used in embodiments as organic binder of thick film paste. The viscosity of such ink, however, is too high to be used in offset printing directly. In order to increase the amount of transfer and enhance the printability, the amount of resin may be increased. However, to increase the high molecular weight resin while maintaining the printability, the amount of solvent must be also increased, so that the pigment concentration in ink may be lowered, and the ink density may drop in the thick film. Or only by increasing the content of the resin, the scattering performance worsens, and the binder is not scattered completely while baking, and it may be left over as carbon particles, which may seriously influence the resistance.